There are a number of different types of semiconductor-based imager devices, including charge coupled devices (CCD's), photodiode arrays, charge injection devices (CID's), hybrid focal plane arrays, and complementary metal oxide semiconductor (CMOS) imagers. Current applications of solid-state imager devices include cameras, scanners, machine vision systems, vehicle navigation systems, video telephones, computer input devices, surveillance systems, auto focus systems, star trackers, motion detector systems, image stabilization systems, and other image acquisition and processing systems.
Solid state imager devices include an array of pixel cells, which converts light energy received, through an optical lens, into electrical signals. Each pixel cell contains a photosensor for converting a respective portion of a received image into an electrical signal. The electrical signals produced by the photosensors in the pixel cells are processed to render a digital image.
In a CMOS imager, the active elements of a pixel cell perform the necessary functions of: (1) photon to charge conversion; (2) accumulation of image charge; (3) transfer of charge to a floating diffusion region; (4) resetting the floating diffusion region to a known state; (5) selection of a pixel cell for readout; and (6) output and amplification of a signal representing the pixel cell charge. Photo-charge may be amplified when it moves from the initial charge accumulation region to the floating diffusion region. The charge at the floating diffusion region is typically converted to a pixel output voltage by a source follower output transistor.
To detect color, the spectral components of incident light must be separated and collected. An absorptive color filter array (CPA) on top of an imager device is currently the dominant technology for color detection in solid state imager devices. In a typical imager device layout, a micro-lens and color filter are stacked as part of a pixel stack.
For example, as shown in FIG. 1, color filters 15 are deposited on top of an array of pixel cells 22 formed on a semiconductor substrate 10. Each pixel cell 22 has a photosensor 12, which may be any photon-to-charge converting device, such as a photogate, photoconductor or photodiode. The color filters 15 are typically formed as an array over one or more optional metal light shield layer 18 in the imager device 20, and are further separated from the photosensor 12 by one or more interlevel dielectric (ILD) and metallization layers 16 and a passivation layer 14. Portions of the metallization layers 16 and the metal shields 18 may be opaque and used to shield the area of the pixel cells 22 that is not intended to be light-sensitive. Convex micro-lenses 21 are typically formed over the color filters 15. In operation, incident light is focused by the micro-lenses 21 through the filter array 15 to the appropriate light-sensitive photosensor 12.
In an effort to reduce the pixel stack height and to bring the micro-lenses 21 and color filters 15 closer to the photosensors 12, the entire color filter array can be lowered into a recessed area within the imager device 20. However, the use of a recessed color filter array has some problems. For example, if the recess depth exceeds the thickness of the color filter array film, the typical method of planarizing, i.e., chemical mechanical planarization (CMP), may no longer improve the planarity of the color filter array, leaving the color filters 15 with an uneven top surface. The surface unevenness can cause problems for an imager device. For example, the uneven surface does not create a solid foundation for a micro-lens array, which is typically constructed over the color filters. In addition, an uneven color filter array surface can cause imaging efficiency reduction by creating additional fixed pattern noise or a shading effect in certain areas of the imager device.
There is needed, therefore, an imager device providing a color filter array within a recessed area such that a lack of planarity in the surface of the color filter array does not degrade the imaging quality of the device.